High-speed scan type x-ray generator

ABSTRACT

A high-speed scan type X-ray generating apparatus for scanning X-ray generating positions along a circumference of an examinee, in which an electron beam is emitted from an electron gun into a ring-shaped vacuum tube. The electron beam is deflected by electromagnets or the like to run on a circular orbit through the vacuum tube. The electron beam is further deflected by different, small electromagnets to deviate from the circular orbit and impinge on a ring-shaped target, thereby generating an X-ray toward the center of the vacuum tube. By controlling the small electromagnets, the X-ray generating position is caused to scan at high speed along a circumferential wall of the ring-shaped target.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to a high-speed scan type X-ray generator suitedfor use with an X-ray CT apparatus, which is capable of a high-speedscan of X-ray emitting positions circumferentially arranged around anexaminee.

(2) Description of the Related Art

The X-ray CT apparatus is typically used to obtain images of X-rayabsorptivity distribution in cross sections of an examinee by emittingX-rays from varied directions through 360 degrees (or 180 degrees)around the examinee, and subjecting the multi-directional X-raytransmission data which is thereby collected to image regenerationprocessing. In order to collect multi-directional X-ray transmissiondata, the X-ray CT apparatus usually has an X-ray tube rotatable by arotating mechanism to emit X-rays from varied directions around anexaminee.

With the rotation of the X-ray tube itself, however, data cannot becollected quickly since it takes about one second for the X-ray tube tomake one complete rotation or a half rotation to obtain a single sliceimage. The above photographic method is therefor not fit for examinationof an organ such as the heart whose movement can be captured only withhigh-speed imaging in the order of 30 frames per second.

In view of the above drawback, an X-ray generator has been proposed inrecent years, which is capable of running an X-ray generating positionon a circumference at a very high speed. This known high-speed scan typeX-ray generator will be described hereunder with reference to FIG. 1.The generator comprises a bell-shaped vacuum tube 1, and an electron gun2 connected to a proximal end of the vacuum tube 1. The vacuum tube 1contains deflecting coils 3, deflecting electrodes 4, and a ring-shapedtarget 5. An electron beam 6 emitted from the electron gun 2 isdeflected by the deflecting coils 3 and deflecting electrodes 4 toimpinge on the target 5. As a result, an X-ray 7 is emitted from thetarget 5 toward a central part of the vacuum tube 1. By controlling thedeflecting coils 3 and deflecting electrodes 4, an X-ray generatingposition (focal point) 8 is caused to run at high speed along thecircumferential wall of the target 5. Consequently, the X-ray 7 isemitted from varied directions around an examinee M, who is introducedinto the central part of the vacuum tube 1. In this way, a picture forone frame can be picked up, for example in about 50 msec.

With this known high-speed scan type X-ray generator, however, theelectron beam 6 is run in the direction perpendicular to a plane formedby the ring-shaped target 5 or by the circumference on which the X-raygenerating position 8 moves, and the electron beam 6 is deflected in thecourse of its run. Consequently, the X-ray generator must have a verylarge size, about 4 meters long in the direction perpendicular to theplane formed by the ring-shaped target 5 (i.e. axially of the examineeM). Therefore, an X-ray CT apparatus using such an X-ray generatorrequires a large installation space.

SUMMARY OF THE INVENTION

This invention has been made with regard to the state of the art notedabove, and its main object is to provide a high-speed scan type X-raygenerator of compact construction having a reduced length axially of anexaminee.

Other objects of this invention will be apparent from the followingdescription.

The above and other objects are fulfilled, according to this invention,by a high-speed scan type X-ray generating apparatus for scanning X-raygenerating positions along a circumference of an examinee, comprising aring-shaped vacuum tube, an electron gun for emitting an acceleratedelectron beam into the vacuum tube, a first deflecting device forcausing the electron beam to run on a ring-shaped orbit through thevacuum tube, a second deflecting device for causing the electron beam todeviate from the ring-shaped orbit, and a target for generating X-raystoward center of the vacuum tube when the electron beam deviating fromthe ring-shaped orbit impinges thereon.

The electron beam may be emitted into the ring-shaped vacuum tube fromone or more electron guns. The electron beam emitted from the electrongun can, for example, enter the ring-shaped vacuum tube tangentially ofthe ring-shaped orbit in the vacuum tube. Where the electron beam entersthe vacuum tube in a direction intersecting the ring-shaped orbit, anadditional deflecting device is used to put the electron beam in thering-shaped orbit.

The first deflecting device may be formed of magnets or electrodes.Where magnets are used, a pair of ring-shaped magnets may be opposed toeach other across the vacuum tube for generating a magnetic fieldperpendicular to a plane formed by the ring-shaped vacuum tube. Thesemagnets may be electromagnets or permanent magnets. The electron beamentering the vacuum tube moves into the circular orbit by the action ofthe magnetic field formed by these magnets.

The electron beam may be converged radially of the circular orbit bymeans of pole faces of the pair of opposite magnets inclined to divergefrom each other as they extend toward the center of the vacuum tube.Where the two pole faces of the magnets are inclined as above, the linesof magnetic force formed between the pole faces become curved, tendingto disperse the electron beam in a direction perpendicular to the planeformed by the circular orbit. It is therefore desirable to converge theelectron beam in the direction perpendicular to the plane formed by thecircular orbit. This may be achieved by forming hills and valleys on theinclined pole faces to alternate high and low flux densities, or byalternately reversing polarity of magnetic poles, in the direction oftravel of the electron beam. In this case, a mean magnetic field formedmust cause the electrons to describe a circular orbit.

The second deflecting device is formed, for example, of at least onepair of small electromagnets disposed in spaces between the oppositepole faces of the magnets acting as the first deflecting device and thevacuum tube, for generating a magnetic field opposite to the magneticfield formed by the magnets. The magnetic field formed by the smallelectromagnet causes the electron beam to deviate radially outwardlyfrom the ring-shaped orbit. Where the target is a ring-shaped targethaving an inside peripheral wall on which the electron beam impinges,after having deviated radially outwardly of the circular orbit, theX-rays travel toward the center of the ring-shaped vacuum tube. Wherethe second deflecting device is formed of a single small electromagnet,the X-ray generating position may be caused to scan the insideperipheral wall of the target at high speed by controlling the value ofcurrent supplied to the small electromagnet. Where the second deflectingdevice includes a plurality of small electromagnets, the X-raygenerating position may be caused to scan the inside peripheral wall ofthe target at high speed by successively switching the smallelectromagnets on and off.

The second deflecting device may have a different construction such asincluding at least one pair of small electromagnets opposed to oneanother across and radially of the vacuum tube. In this case, a magneticfield opposite to the magnetic field formed by the magnets is formed tocause the electron beam to deviate in a direction intersecting the planeformed by the vacuum tube. The target used in this case is a ring-shapedtarget having a wedge-shaped section for generating the X-rays towardthe center of the vacuum tube when the electron beam deviating from thecircular orbit impinges thereon.

Further, the second deflecting device may be formed of a ring-shapedfixed cathode and a ring-shaped grid mounted inside the ring-shapedvacuum tube. The target in this case is a ring-shaped target opposed tothe fixed cathode across the grid. By varying the voltage applied to thegrid, the X-ray generating position may be caused to scan thecircumferential wall of the target at high speed.

According to this invention, as described above, X-rays may be emittedfrom various positions in the ring-shaped vacuum tube, and the X-raygenerating position may be caused to scan at high speed. The compactconstruction provided by this invention has a great advantage withregard to installation space.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there are shown in thedrawings several forms which are presently preferred, it beingunderstood, however, that the invention is not limited to the precisearrangements and instrumentalities shown.

FIG. 1 is a view in vertical section showing an outline of aconventional high-speed scan type X-ray generator.

FIG. 2 is a plan view of an apparatus in a first embodiment of thisinvention.

FIG. 3 is a cut away section taken on line A--A of FIG. 2.

FIG. 4 is a sectional view showing modified first and second deflectingdevices.

FIG. 5 is a sectional view showing another modified second deflectingdevice.

FIG. 6 is a plan view showing an example in which a vacuum tube includesa plurality of accelerating electrodes.

FIG. 7 is a sectional view showing a principal portion of an apparatusin a second embodiment of this invention.

FIGS. 8 through 10 are views illustrating functions of the secondembodiment.

FIGS. 11 and 12 are explanatory views of a modification of the secondembodiment.

FIG. 13 is a plan view of an apparatus in a third embodiment.

FIG. 14 is a section taken on line B--B of FIG. 13.

FIG. 15 is a section taken on line C--C of FIG. 13.

FIG. 16 is a section taken on line D--D of FIG. 13.

FIG. 17 is a section taken on line E--E of FIG. 13.

FIG. 18 is a section taken on line F--F of FIG. 13.

FIG. 19 is a fragmentary perspective view of a ring-shaped grid and aring-shaped target.

FIG. 20 is a view showing an electric connection structure of theapparatus in the third embodiment.

FIG. 21 is a view showing a waveform of voltage applied to the grid.

FIG. 22 is a view illustrating functions of the third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of this invention will be described in detailhereinafter with reference to the drawings.

FIRST EMBODIMENT

FIG. 2 is a plan view of a high-speed scan type X-ray generatoraccording to a first embodiment of the invention. FIG. 3 is a sectiontaken on line A--A of FIG. 2.

This high-speed scan type X-ray generator includes a ring-shaped vacuumtube 11 defining a hollow space in the center thereof for receiving anexaminee M. An electron gun 12 is connected to the vacuum tube 11, whichincludes a filament 12a for emitting an electron beam 6, andaccelerating electrodes 12b for accelerating the electron beam 6 priorto entry to the vacuum tube 11. In order to cause the incident electronbeam 6 to run along a circular orbit OR1 as shown in FIG. 2, tworing-shaped electromagnets 13 are arranged opposite upper and lowersurfaces of the vacuum tube 11, respectively, as shown in FIG. 3. Eachof the electromagnets 13 includes a ring-shaped core 13a and a coil 13bwound thereon. These electromagnets 13 correspond to a first deflectingdevice of this invention. A uniform magnetic field is formed betweenthese electromagnets 13 in a direction perpendicular to a plane formedby the ring-shaped vacuum tube 11, i.e. in a direction from the upperelectromagnet 13 to the lower electromagnet 13. Assuming, for example,that the electrons have an energy of 100 keV and the circular orbit OR1has a diameter about 0.6 m, the pair of electromagnets 13 may form amagnetic field of about 37 gauss therebetween.

Small electromagnets 14 are disposed in spaces between theelectromagnets 13 and vacuum tube 11, in pairs opposed to one anotheracross the vacuum tube 11. Such pairs of small electromagnets 14 arearranged equidistantly along the vacuum tube 11. These smallelectromagnets 14 constitute a second deflecting device of thisinvention. Each pair of opposed small electromagnets 14 forms a magneticfield having an opposite direction to the magnetic field formed by thering-shaped electromagnets 13 (i.e. the direction from the lower smallelectromagnet 14 to the upper small electromagnet 14 in FIG. 3). Thepairs of small electromagnets 14 arranged along the vacuum tube 11 areturned on and off individually.

When the small electromagnets 14 are off, the electron beam 6 enteringthe vacuum tube 11 moves along the circular orbit OR1. When a certainpair of the small electromagnet 14 is turned on, the magnetic fieldthereby formed applies a force to the electron beam 6, whereby theelectron beam 6 deviates from the circular orbit OR1 to follow an orbitswerving outwardly of the circular orbit OR1 (i.e. an orbit OR2 in FIGS.2 and 3).

The vacuum tube 11 contains a ring-shaped target 15 extending along anoutward wall thereof. The abovementioned orbit OR2 intersects the target15, and therefore the electron beam 6 following the orbit OR2 impingeson the target 15. As a result, an X-ray is generated at a position ofimpingement to travel inwardly, i.e. toward the center, of thering-shaped vacuum tube 11.

Thus, by turning on any one of the plural pairs of small electromagnets14, the electron beam 6 may be caused to deviate from a selectedposition of the circular orbit OR1 for impingement on the target 15. Byhigh-speed switching of the current for energizing the smallelectromagnets 14, the impinging position of electrons, i.e. X-raygenerating position (focal point), may be shifted at high speed alongthe inside wall of the target 15. Fine control may be made of the X-raygenerating position by arranging the small electromagnets 14 in highconcentration along the ring-shaped vacuum tube 11.

Where the electron beam 6 is allowed to impinge on the target 15 atvaried angles thereto, the position of the target 15 on which theelectron beam 6 impinges may be controlled by adjusting the intensity ofthe magnetic fields formed by the small electromagnets 14. Forcontrolling the X-ray generating position by means of the magnetic fieldintensity, the small electromagnets 14 may be reduced in number and asingle pair of such magnets will serve the purpose.

In the foregoing embodiment, plural pairs of small electromagnets 14 areprovided to form the magnetic fields for orbit deviation. Alternatively,part of the magnetic field formed by the ring-shaped electromagnets 13may be nullified, through which the electron beam 6 will departtangentially from the circular orbit OR1 to impinge on the target 15.Thus, as shown in FIG. 4, divided electromagnets 16 may be arrangedalong the upper and lower surfaces of the vacuum tube 11. In thisconstruction, the respective pairs of upper and lower electromagnets 16are successively switched on and off, such that the magnetic fields areformed upstream and not downstream of a certain position with respect toa traveling direction of the electron beam 6. Consequently the X-raygenerating position is caused to run at high speed along the inside wallof the target 15.

Further, in the foregoing embodiment, the X-ray generating ring-shapedtarget 15 is disposed outwardly of and concentrically with the circularorbit OR1 of the electron beam 6. The target 15 may be disposed eitherupwardly or downwardly inside the vacuum tube 11. As shown in FIG. 5,for example, a target 17 having a wedge-shaped section may be disposeddownwardly inside the vacuum tube 11. In this case, the smallelectromagnets 14 are arranged along the inward wall and outward wall ofthe vacuum tube 11 to be opposed to one another across the vacuum tube11. The small electromagnets 14 form magnetic fields from radiallyinwardly to outwardly of the vacuum tube 11 to direct the electron beam6 to the target 17.

In the foregoing embodiment, the magnetic field formed by thering-shaped electromagnets 13 is used to cause the electron beam 6 torun along the circular orbit OR1, and the magnetic fields formed by thesmall electromagnets 14 are used to cause the electron beam 6 to deviatefrom the circular orbit OR1. These electromagnets may be replaced withelectrodes to effect a similar control by means of electric fieldsthereby formed.

The electron gun 12 may comprise the type that emits a beam of electronscontinuously or the type that emits the beam intermittently. Theelectron gun 12 has a reduced load when emitting the electron beamintermittently.

The X-rays generated may be given variable energy by varying theelectron beam accelerating energy while maintaining its correlation withthe magnetic or electric field that causes the electron beam to runalong the circular orbit OR1.

In the foregoing embodiment, the accelerating electrodes 12b arearranged only adjacent the filament 12a. As shown in FIG. 6, thering-shaped vacuum tube 11 may include additional acceleratingelectrodes 18a-18c disposed at an appropriate position or positionsthereof for re-accelerating the electron beam, thereby to compensate forenergy loss of the electron beam. This construction allows the electronbeam enclosed in the ring-shaped vacuum tube 11 to continue moving alongthe circular orbit OR1. The load of the electron gun 12 may thereby bereduced further.

The foregoing embodiment has been described as deflecting the electronbeam to move along the circular orbit OR1. However, an elliptical orpolygonal orbit of the electron beam is also conceivable. In the case ofa polygonal orbit, magnets or electrodes are disposed adjacent therespective vertices to form magnetic or electric fields for deflectingthe beam.

SECOND EMBODIMENT

A second embodiment of this invention will be described next.

With the high-speed scan type X-ray generator in the first embodiment,the electron beam tends to be dispersed radially of the circular orbitOR1 owing to non-uniformity or space charge effect of the magnetic fieldwhen large quantities of electrons impinge on parallel pole faces(referenced 13c in FIG. 3) of the pair of ring-shaped electromagnets 13.When the electron beam is dispersed, the focal point of the X-ray isenlarged to deteriorate quality of the images picked up by X-ray CT.This second embodiment provides an improvement for eliminating thisdrawback of the first embodiment as explained below.

FIG. 7 is a sectional view corresponding to FIG. 3 of the firstembodiment. In FIG. 7, like reference numerals are used to identify likeparts in FIG. 3 which are the same as in the first embodiment, andtherefore will not be described again.

As shown in FIG. 7, the characterizing feature of this embodiment liesin electromagnets 20 arranged opposite the upper and lower surfaces ofthe vacuum tube 11. Each of these electromagnets 20 includes a core 20adefining an outwardly projecting flange, and a coil 20b wound around thecore 20a. The cores 20a define opposed pole faces 20c which are inclinedto diverge from each other as they extend toward the center of the ring.

Reference is now made to FIG. 8 for illustrating the way in which theelectron beam runs through the magnetic flux formed between the opposedpole faces 20c of the electromagnets 20. The electron beam 6, whichenters the magnetic flux formed between the pole faces 20c, is subjectedto the force of the flux acting perpendicular to the running directionof the electron beam 6 and to the direction of the flux (that is, inFIG. 8, rightward on the assumption that the electron beam 6 runs atright angles to the sheet of drawings from front to back). As a result,the electron beam 6 runs on a circular orbit having a radius Ro. Thatis, the electron beam 6 receives Lorentz's force F1 expressed by thefollowing equation:

    F1=evB

where e is an electric charge of the electrons, v is a velocity thereof,and B is a flux density. On the other hand, the centripetal force F2 ofthe electrons running on this circular orbit is expressed by thefollowing equation:

    F2=mv.sup.2 /R

where m is the mass of the electrons and R is the radius of the circularorbit. With these forces in equilibrium, i.e.

    F1=F2,

and with the flux density B, the electrons are caused to run on thecircular orbit having radius R. Thus,

    evB=mv.sup.2 /R.

Therefore,

    BR=mv/e.

The right side of the equation takes a fixed value unless the kineticenergy (mv² /2) of the electrons changes. Thus, the orbit radius R isfixed if the flux density is fixed.

If the flux density B at the position of radius Ro shown in FIG. 8 is;

    BRo=C (constant),

the flux density becomes less (B-ΔB) in the regions closer to the centerO since the pole faces 20c are wider apart from each other.Consequently, for the electrons passing through the regions inwardly ofthe position of radius Ro,

    R=C/(B-ΔB)>Ro,

and the electrons move outwardly away from the center O. Conversely, forthe electrons passing through the regions outwardly of the position ofradius Ro,

    R=C/(B+ΔB)<Ro,

and the electrons move inwardly toward the center O. As a result, theelectron beam 6 converges to the position of radius Ro.

As shown in FIG. 9, the pole faces 20c define hills and valleys arrangedin opposed relations in the running direction of the electron beam 6,i.e. circumferential direction. Consequently, the pole faces 20calternate between being close to and being remote from each other. Sincethe pole faces 20c diverge from each other as they extend inwardly, thelines of magnetic force become curved as shown in FIG. 10, therebygenerating forces to disperse, in the direction of arrow Y, theelectrons that are out of a plane (shown in a broken line in FIG. 10)midway between the pole faces 20c. The above structure is employed tosuppress such dispersion of the electrons. The hills and valleys formedon the pole faces 20c provide narrow regions having an increased fluxdensity (B+B1) and broad regions having a decreased flux density (B-B1),which alternate n times in one circle (360 degrees). This structure hasthe effect, based on the principle of cyclotron strong convergence, ofconverging the electron beam 6 in the Y direction with running of theelectron beam 6.

Apart from the hills and valleys formed on the pole faces 20c,dispersion in the Y direction of the electron beam 6 may be suppressedalso by the following structure. As shown in FIG. 11, a plurality ofmagnets 19 with magnetic poles reversing alternately in thecircumferential direction are arranged in the spaces between thering-shaped vacuum tube 11 and the electromagnets 20 defining oppositepole faces 20c inclined to diverge from each other as they extend towardthe ring center. These magnets 19 may be electromagnets or permanentmagnets. FIG. 12 illustrates magnetic fields formed by theelectromagnets 20 and magnets 19. The dispersion in the Y direction ofthe electron beam 6 may also be suppressed by the alternate reversal ofpolarity in the circumferential direction. It is necessary, however, toset a mean magnetic field between the pole faces 20c to an intensitywhich will cause the electrons to describe a circular orbit.

As described above, the electron beam 6 may be converged by providingthe electromagnets 20 opposed to each other across the vacuum tube 11 toform a magnetic field for causing the electron beam 6 to move along acircular orbit, and appropriately shaping the pole faces 20c oralternately reversing the magnetic polarity.

When transmitting a large amount of electrons in acceleration as notedabove, the electron beam 6 usually becomes dispersed out of a fixedtrack owing to non-uniformity of the magnetic field, space charge effector other factors. It is therefore difficult to obtain a beam of a largeamount of electrons; the beam must be converged by forming additionalelectric or magnetic fields. This would result in a large andcomplicated construction of the apparatus. However, a small and simpleapparatus may be realized at low manufacturing cost by appropriatelyshaping the pole faces 20c of the electromagnets 20 or alternatelyreversing magnetic polarity.

The function of the small electromagnets 14 to cause the electron beam 6entering the vacuum tube 11 to deviate from the circular orbit OR1 andcollide with the target 15 is the same as in the first embodiment, andtherefore is not described again.

THIRD EMBODIMENT

FIG. 13 is a plan view showing an outline of a third embodiment of thisinvention.

This X-ray generator comprises a ring-shaped vacuum tube 21 defining ahollow space in the center for receiving an examinee M, as in the firstembodiment. Two electron guns 22 are connected to the vacuum tube 11.Each of the electron guns 22 includes a filament 22a for emitting anelectron beam 6, and accelerating electrodes 22b for accelerating theelectron beam 6.

The accelerated electron beam 6 enters the vacuum tube 21, and,immediately upon entry, is deflected by a magnetic field function ofdeflecting magnets 23. These deflecting magnets 23 form a deflectingmagnetic field to put the incident electron beam 6 in a circular orbitalong the ring-shaped vacuum tube 21. As shown in FIG. 14, thedeflecting magnets 23 are interconnected through a ferromagnetic yoke24. The magnetic field formed by the deflecting magnets 23 (whichmagnetic field extends from back to front with respect to the plane ofFIG. 13) deflects the electron beam 6 entering the vacuum tube 21leftward with respect to the running direction thereof, whereby theelectron beam 6 runs circumferentially along the vacuum tube 21b.

The vacuum tube 21 has coils 25 extending along the vacuum tube 21 asshown in FIGS. 14 through 18, to form a magnetic field for moving theelectron beams 6 along the circular orbit. These coils 25 have afunction equivalent to that of the ring-shaped electromagnets 13 in thefirst embodiment, and form a magnetic field uniformly in thecircumferential direction of the vacuum tube 21. This magnetic fieldextends from front to back with respect to the plane of FIG. 13 (whichis shown in broken lines in FIGS. 15 through 18). Consequently, theelectron beams 6 deflected by the deflecting magnets 23 invariably aresubjected to forces acting rightward with respect to the runningdirection thereof (i.e. toward the center of the ring-shaped vacuum tube21). The electron beams 6 are thus caused to move along the circularorbit substantially coaxial with the ring-shaped vacuum tube 21 byadjusting a current flowing through the coils 25 to appropriately setintensity of this magnetic field.

As shown in FIGS. 14 through 18, the vacuum tube contains a ring-shapedfixed cathode 26, a ring-shaped grid 27 and a ring-shaped target 28 (seeFIG. 19 also). The fixed cathode 26 and grid 27 correspond to the seconddeflecting device of this invention. These components are all formedsubstantially coaxially with the ring-shaped vacuum tube 21, and arearranged in a direction perpendicular to the plane formed by the vacuumtube 21, i.e. axially of the examinee M. As shown in FIG. 29, the grid27 includes a mesh portion 27a in the center thereof. As shown in FIGS.13 and 18, these electrodes 26, 27 and 28 are connected at a voltagesupply position 29 to cables 30 and 31 for application of voltages.

FIG. 20 shows electric connections for the fixed cathode 26, grid 27 andtarget 28, and the filament 22a and accelerating electrodes 22b of eachelectron gun 22. A sawtooth deflecting voltage source 32 is connectedbetween the fixed cathode 26 and grid 27, and an electron orbitdeflecting high voltage source 33 is connected between the fixed cathode26 and target 28.

FIG. 21 shows a sawtooth deflecting voltage applied to the grid 27. Whenthis grid voltage is high, the electron beam 6 emitted from eachelectron gun 22 and deflected by the deflecting magnets 23 to runthrough a space between the fixed cathode 26 and grid 27 is drawn towardthe grid 27 by a strong electrostatic force. Consequently, the electronbeam 6 impinges on the target 28 after passing through the grid 27 at anearly stage, i.e. at a position close to the electron gun 22. On theother hand, when the grid voltage is low, only a weak electrostaticforce is operative to draw the electron beam 6 toward the grid 27.Consequently, each electron beam 6 passes through the grid 27 at aposition remote from the electron gun 22 to reach the target 28. Whenthe electron beam 6 impinges on the target 28, as shown in FIG. 17, anX-ray 7 is generated at the position of impingement and travelstherefrom toward the center of the ring-shaped vacuum tube 21, i.e.toward the examinee M.

This embodiment includes two electron guns 22 located 180 degrees apartfrom each other. Thus, the X-ray generating position may be movedthrough 360 degrees by causing the electron beam 6 emitted from eachelectron gun 22 to impinge on the target 28 through the 180 degreerange. In the example shown in FIG. 13, the electron beam 6 emitted fromthe left electron gun 22 covers the upper right range from point a topoint d, while the electron beam 6 emitted from the right electron gun22 covers the lower left range from point d to point a. For thispurpose, the grid voltage shown in FIG. 21 is at a maximum Va when theelectron beam 6 emitted from the left electron gun 22 reaches the target28 at point a, and the electron beam 6 emitted from the right electrongun 22 reaches the target 28 at point d. The grid voltage is at aminimum Vd when the electron beam 6 emitted from the left electron gun22 reaches the target 28 at point d, and the electron beam 6 emittedfrom the right electron gun 22 reaches the target 28 at point a. Whenthe grid voltage is at Vb, the electron beam 6 emitted from the leftelectron gun 22 reaches the target 28 at point b. When the grid voltageis at Vc, the electron beam 6 emitted from the left electron gun 22reaches the target 28 at point c and the electron beam 6 emitted fromthe right electron gun 22 reaches the target 28 at point c'.

FIGS. 22 shows tracks Ta, Tb, Tc and Td followed by the electron beam 6emitted from the left electron gun 22 when the grid voltage is Va, Vb,Vc and Vd, respectively. In this graph, the horizontal axis representsthe circumferential direction of the ring-shaped vacuum tube 21, and thevertical axis the axial direction of the vacuum tube 21 (i.e. the axialdirection of the examinee M), that is positions at which the electronbeam 6 travels from the fixed cathode 26 to the target 28. It will beseen that, by varying the grid voltage from Va to Vd, the electron beam6 is caused to take varied tracks as shown in FIG. 22, thereby to movethe X-ray generating position through the 180 degree range from point ato points b, c, and d. Where the sawtooth grid voltage has cycles of 10msec, the X-ray generating position will complete a scan through the 180degree range in 10 msec.

The foregoing positional relationship among the fixed cathode 26, grid27 and target 28 in the ring-shaped vacuum tube 21 is illustrated by wayof example only. These electrodes 26, 27 and 28 may be arranged radiallyof the vacuum tube 21 a in the first embodiment.

The number of electron guns 22 is not limited to two, but may be one,three or more. Electrons may be emitted from a plurality of electronguns simultaneously to generate X-rays at the corresponding number ofpositions simultaneously, or may be emitted with time lags.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, for determining the scope of theinvention.

What is claimed is:
 1. A high-speed scan type X-ray generating apparatusfor scanning X-ray generating positions along a circumference of anexaminee, said apparatus comprising;a ring-shaped vacuum tube, at leastone electron gun for emitting an accelerated electron beam into saidvacuum tube, first deflecting means for causing said electron beam torun on a ring-shaped orbit through said vacuum tube, said firstdeflecting means including a pair of ring-shaped magnets oppose to eachother across said vacuum tube for generating a magnetic fieldperpendicular to a plane formed by said ring-shaped vacuum tube; seconddeflecting means for causing said electron beam to deviate from saidring-shaped orbit, said second deflecting means includes at least onepair of small electromagnets disposed in spaces between opposite polefaces of said ring-shaped magnets and said vacuum tube, for generating amagnetic field opposite to said magnetic field formed by saidring-shaped magnets, to cause said electron beam to deviate radiallyoutwardly from said ring-shaped orbit; and a target for generatingX-rays toward center of said vacuum tube when said electron beamdeviating from said ring-shaped orbit by said second deflecting means,impinges thereon, said target being a ring-shaped target having aninside peripheral wall for generating the X-rays toward the center ofsaid vacuum tube; wherein opposite pole faces of the pair of ring-shapedmagnets constituting said first deflecting means are inclined to divergefrom each other toward the center of said ring-shaped vacuum tube.
 2. Ahigh-speed scan type X-ray generating apparatus for scanning X-raygenerating positions along a circumference of an examinee, saidapparatus comprising:a ring-shaped vacuum tube; at least one electrongun for emitting an accelerated electron beam into said vacuum tube;first deflecting means for causing said electron beam to run on aring-shaped orbit through said vacuum tube, said first deflecting meansincludes a pair of ring-shaped magnets opposed to each other across saidvacuum tube for generating a magnetic field perpendicular to a planeformed by said ring-shaped vacuum tube; second deflecting means forcausing said electron beam to deviate from said ring-shaped orbit, saidsecond deflecting means including at least one pair of smallelectromagnets opposed to one another across said radially of saidvacuum tube for generating a magnetic field opposite to said magneticfield formed by said ring-shaped magnets, to cause said electron beam todeviate in a direction intersecting said plane formed by said vacuumtube; and a target for generating X-rays toward a center of said vacuumtube when said electron beam, after being deviated from said ring-shapedorbit to said second deflecting means, impinges thereon, said target isa ring-shaped target having a wedge-shaped section for generating theX-rays toward the center of said vacuum tube; wherein opposite polefaces of the pair of ring-shaped magnets constituting said firstdeflecting means are inclined to diverge from each other toward thecenter of said ring-shaped vacuum tube.
 3. An apparatus as claimed inclaim 1 or 2, wherein said ring-shaped vacuum tube contains at least oneaccelerating electrode disposed along the base orbit for acceleratingsaid electron beam, in addition to the accelerating electrodes forcausing the electron beam emitted from the electron gun to enter thevacuum tube.
 4. An apparatus as claimed in claims 1 or 2, wherein saidopposite pole faces of said ring-shaped magnets define hills and valleysarranged in a direction of travel of said electron beam and opposed toone another.
 5. An apparatus as claimed in claims 1 or 2, furthercomprising a plurality of magnets arranged between said opposite polefaces of said ring-shaped magnets and having polarities alternatelyreversed in a circumferential direction.